Circle of the Moon was made by another team, it's the only one that wasn't produced by IGArashi. Funny, they fixed the weak points of SOTN (low difficulty and easy bosses) but they messed ALL the rest of things xD The castle layout loss all the personality and there isn't much customization. It lacks the flavor and atmosphere of the IGAvanias, it just feel generic... All the music are rehashes that don't fit the places and so... Too much focused in wandering without finding nothing too useful.
Harmony was the first true sequel to SOTN by IGA. The problem is that they were too ambitious without understanding well the GBA hardware. Well, it's simply uninspired and rushed, but you can see that the atmosphere get a lot more of attention than in Circle of the Moon.
But who cares? Both are forgettable titles in the series. I repeat, Castleroids are all about SOTN, Sorrow games and Ecclesia. Every Castlevania is different and has it own flavour, anyway, even the worse ones. It's not like Megaman, which are obvious romhacks xd